Characterizing Stalagmites’ Eigenfrequencies by Combining In Situ Vibration Measurements and Finite Element Modeling Based on 3D Scans

Aurélie Martin,Thomas Lecocq,Klaus-G Hinzen,Thierry Camelbeeck,Nathalie Fagel,Yves Quinif

doi:10.3390/geosciences10100418

Aurélie Martin, Thomas Lecocq + Show 4 more

Open Access

https://doi.org/10.3390/geosciences10100418

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Abstract

Broken or deformed speleothems have been used as indicators of paleo-earthquakes since the 1990s; however, a causal link is difficult to prove except for some thin speleothems. In contrast, the presence of intact speleothems permits estimating an upper limit of the level of horizontal ground motions of past seismicity in the area. The natural frequencies of speleothems are fundamental parameters for their response to earthquakes. This study proposes a new method of in situ characterization of these natural frequencies. Tested in the Han-sur-Lesse cave (Belgian Ardennes), the method is based on recording the ambient seismic noise using three-component sensors on a stalagmite and a 3D laser scan of its shape. The ambient seismic noise records allow a precise determination of the eigenfrequencies of the stalagmite. In addition, numerical models based on the 3D scan show good consistency between measured and modeled data. The joint analysis of these two techniques concludes that the shape of the stalagmite (elliptical cross-section and shape irregularities) influence the eigenfrequencies and polarization of the modes while also causing a near-orthogonal split of natural frequencies. The motions recorded on the stalagmite show significant amplification compared to those recorded at the free surface outside the cave, which has a strong impact on seismic hazard assessment based on speleothems.

Highlights

The observation and dating of broken or deformed speleothems have been used as indicators of paleo-earthquakes since the 1990s
The natural frequencies of the stalagmites could be identified from the ambient seismic noise and were stable over the entire measurement period
Are larger compared to the motions measured at its base (SOIL) and at the surface outside the cave (SURF); there is no evidence for the nonlinearity of the amplification (Figure 4)

Summary

Introduction

The observation and dating of broken or deformed speleothems have been used as indicators of paleo-earthquakes since the 1990s (see [1,2] for a review). Numerical models and physical experiments pointed out the limits of testing the earthquake hypothesis to explain broken speleothems [9,10] except for very thin stalagmites or stalactites (such as soda straws). On another hand, so far, such models do not take into account all possible imperfections and weaknesses in the body of the speleothems. Broken speleothems can be used to date ceiling collapses that sometimes can be related to paleo-earthquakes [11].

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